伊藤 洋一

Abstract In this study, we revisit 32 planetary systems around evolved stars observed within the framework of the Okayama Planet Search Program (OPSP) and its collaborative framework of the East Asian Planet Search Network to search for additional companions and investigate the properties of stars and giant planets in multiple-planet systems. With our latest radial velocities obtained from Okayama Astrophysical Observatory, we confirm an additional giant planet in the wide orbit of the 75 Cet system ($P_{\rm {c } } = 2051.62_{-40.47}^{+45.98}\ \rm {d}$, $M_{\rm {c } }\sin i=0.912_{-0.090}^{+0.088}\,\,M_{\rm {J } }$, and $a_{\rm {c } }=3.929_{-0.058}^{+0.052}\ \rm {au}$), along with five stars exhibiting long-term radial velocity accelerations, which indicates massive companions in the wide orbits. We also find that the radial velocity variations of several planet-harboring stars may indicate additional planet candidates, stellar activities, or other understudied sources. These stars include ϵ Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and ω Ser. We further constrain the orbital configuration of the HD 5608, HD 14067, HD 120084, and HD 175679 systems by combining radial velocities with astrometry, as their host central stars exhibit significant astrometric accelerations. For other systems, we simply refine their orbital parameters. Moreover, our study indicates that the OPSP planet-harboring stars are more metal poor compared to the currently known planet-harboring stars, and this is likely due to the B − V color upper limit at 1.0 for star selection in the beginning of the survey. Finally, by investigating the less massive giant planets (<5 MJ) around currently known planet-harboring evolved stars, we find that metallicity positively correlates with the multiplicity and total planet mass of the system, which can be evidence for the core-accretion planet formation model.

We report the detection of a giant planet orbiting the G-type giant star HD 167768 from radial velocity measurements using the High Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory (OAO). HD 167768 has a mass of 1.08(-0.12) (+0.14) M-?, a radius of 9.70 R-+0.25 (-0.25)?, a metallicity of [Fe/H] = -0.67+0.09 -0.08 , and a surface gravity of log g = 2.50(-0.06.)(+0.06) .The planet orbiting the star is a warm Jupiter, having a period of 20.6532(-0.0032)( +0.0032) d, a minimum mass of 0.85(-0.11)(+0.12) MJ , and an orbital semimajor axis of 0.1512 ( +0.0058)(-0.0063)au. The planet has one of the shortest orbital periods among those ever found around deeply evolved stars (log g < 3.5) using radial velocity methods. The equilibrium temperature of the planet is 1874 K, as high as a hot Jupiter. The radial velocities show two additional regular variations at 41 d and 95 d, suggesting the possibility of outer companions in the system. Follow-up monitoring will enable validation of the periodicity. We also calculated the orbital evolution of HD 167768 b and found that the planet will be engulfed within0.15 Gyr.